In the technical field of displays, due to the advantages of lightness, thinness, low power consumption, high brightness and high image quality, etc., flat panel display devices, for example, Liquid Crystal Display (LCD) and Organic Light Emitting Display (OLED), take an important position in the field of flat panel displays.
Wherein, the property of a Thin Film Transistor (TFT), which functions as a pixel switching apparatus, plays an important role in a high-image quality display device.
The operating principle of a TFT as a switching apparatus is outlined as follows: when a voltage Vg relative to ground voltage (gate electrode voltage for short) is applied to a gate electrode of a TFT, an electric field will be generated between the gate electrode of the TFT and a drain electrode connected with a data line, and an conducting area formed in TFT under the action of the electric field makes a source electrode connected with a pixel electrode switch into conduction with the drain electrode via the semiconductor layer of the TFT, thus the larger the forward voltage difference (i.e., threshold voltage Vth) between the gate electrode and the source electrode is and the wider the conducting area will be, the larger the on-state current will be and the stronger the charging capacity on the pixel electrode will be; when a negative voltage relative to ground voltage is applied to the gate electrode of the TFT, the source electrode and the drain electrode will be switched off. This is the switching property of a TFT. The higher the charging capacity for the pixel electrode is, the more thoroughly the pixel electrode will be charged up and discharged, and thereby the image quality can be improved.
Thus, it can be seen that the higher the electron mobility of the semiconductor layer is and the wider the conducting area will be, the larger the on-state current will be.
At present, the on-state current of a TFT may be increased remarkably by using a metal oxide as the TFT semiconductor layer, because the electron mobility of a metal oxide semiconductor layer is 5˜10 times of that of an amorphous silicon semiconductor layer. When the material of the semiconductor layer is determined, the conducting performance (which mainly refers to on-state current, here) of the TFT will depend on the structure of the TFT.
Referring to FIG. 1, it shows a schematic top view of an existing TFT, which includes: a source electrode 15, a drain electrode 16, a gate electrode 11, a scanning line 17 connected with the gate electrode 11 and a data line 18 connected with the drain electrode 16.
Wherein, the number of the source electrode 15 and the drain electrode 16 on the TFT is one separately.
When the TFT is switched on, an on-state current flows from the drain electrode 16 to the source electrode 15 via the conducting area, the distance between the drain electrode 16 and the source electrode 15 determines the length of the conducting area, and the size of the drain electrode 16 and the source electrode 15 determines the width of the conducting area, in a pixel TFT, because the diameters of the drain electrode and the source electrode are usually on the magnitude of several micrometers, the patterns of the source electrode and the drain electrode are usually circles with a certain diameter, or equivalently, squares with a certain side length; in order to improve the performance of the TFT and increase the width of the conducting area, the areas of the source electrode and the drain electrode may be increased, but the following problem will be caused when the area of the source electrode is increased:
When the areas of the source electrode and the drain electrode come to be increased, the area of the subpixel region occupied will also come to be increased, and the source electrode is connected with the pixel electrode of the TFT, thus the aperture ratio of the TFT will come to be decreased, especially when the area of the source electrode is larger. More importantly, when the area of the source electrode grows large, the capacitance between the source electrode and the semiconductor layer will grow large, which may cause color difference of an image displayed in the case that the voltage of the pixel electrode jumps (wherein color difference refers to the deviation between the displayed color and theoretical color of each pixel).
For the existing TFT, when a smaller source electrode capacitance is guaranteed, the conducting area will be smaller, the on-state current of the TFT will be smaller, and thus the performance of the TFT will be poorer.